Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector for each cylinder to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Further, engines have been proposed using more than one type of fuel injection. For example, the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. are one example. Specifically, the Heywood et al. papers describes directly injecting ethanol to improve charge cooling effects, while relying on port injected gasoline for providing the majority of combusted fuel over a drive cycle. The ethanol provides increased charge cooling due to its increased heat of vaporization compared with gasoline, thereby reducing knock limits on boosting and/or compression ratio. Further, water may be included in the mixture. The above approaches purport to improved engine fuel economy and increase utilization of renewable fuels.
However, the inventors herein have recognized a disadvantage with such an approach when the engine combustion chamber may receive varying ratio of fuel types. For example, under conditions where knock limits on spark advance are not restrictive, the cylinders may operate with a lower alcohol amount, whereas under conditions where knock limits on spark advance may cause fuel economy losses, the cylinders may operate with a higher alcohol amount to suppress knock and reduce limits on spark advance. In such cases, a higher temperature spark plug design may cause pre-ignition during the conditions of increased alcohol. Alternatively, a lower temperature spark plug design may cause spark plug fouling during the conditions of decreased alcohol.
In other words, the selection of spark plug heat range is a trade-off between the risk of preignition at high loads and the risk of spark plug carbon fouling at light loads. The proposed combination of ethanol at high loads and gasoline at low loads, for example, makes this trade-off much more difficult, because ethanol is more prone to preignition than gasoline, and gasoline is more prone to spark plug carbon fouling than ethanol.
As such, the inventors herein have recognized an approach to address the above competing spark plug requirements. The approach may include an engine system, comprising a combustion chamber; a first spark plug configured to perform a spark within the combustion chamber, said first plug configured to have a first heat range; a second spark plug configured to produce a spark within the combustion chamber; said second plug configured to have a second heat range different from said first heat range of said first plug; and a delivery system configured to deliver a hydrocarbon fuel and a fluid including an alcohol to the combustion chamber in varying resulting ratios.
By using spark plugs with different heat ranges or configurations, the combustion chamber system can accommodate different fuel/alcohol ratios, while also addressing pre-ignition and spark plug fouling. Thus, the engine control system can select the appropriate spark plug or spark plugs for the operating conditions and fuel delivery strategy, thereby improving engine performance and efficiency, under some conditions.